A wide variety of different types of exercise equipment has been developed in the past, both in the professional and consumer markets. Resistance exercise machines have incorporated weights, deformable resistance elements, and friction or centrifugal mechanisms to provide a resistive force. Usually, resistance exercise equipment takes the form of large stationary machines. Equipment that uses weights as the resistance is obviously heavy and unsuitable for uses where light weight is essential or where portability is important.
U.S. Pat. No. 4,944,511 to Francis discloses a portable machine in which resistance is provided by a variable number of stacked spring packs. Each pack has a known resistance, and the resistance packs are connected together in a manner to provide an additive force. The stacked resistance packs all rotate together so that in applications where a long extension of the cord is required, the resistance packs rotate a significant amount. When the springs approach their limit of deformation, they provide significantly increased resistance. Thus, the resistance can increase sharply during the range of motion of equipment, particularly when the cord nears a fully extended position.
Resistance elements of the type shown in U.S. Pat. No. 5,209,461 to Whightsil can be incorporated in a light weight unit which nevertheless provides considerable resistance. Therefore, this type of resistance pack has characteristics making it suitable for use in exercise equipment where light weight and portability are important. However, achieving a flat resistance-extension curve with this type of resistance pack is still difficult, particularly if the requirements include providing a large resistance force and at the same time accommodating a long extension of the cord or other actuator.
In recent years, it has been discovered that personnel stationed in a micro-gravity environment, as on a space station, tend to lose muscle mass and bone density quickly because of the absence of gravity opposing their normal movements. It is important for such personnel to have resistance exercise equipment available to allow them to exercise regularly and counteract the tendency for muscular atrophy and loss of bone density. Heavy weights are obviously not a viable option for a micro-gravity application of this type. Compactness, light weight and portability are attributes that are necessary for exercise devices intended for use on space stations and similar environments. At the same time, a large resistance force is desirable so that the musculo-skeletal system of crew members can be loaded to provide the required counter measures.